1. Field of the Invention
The present invention relates to a toner, a process cartridge, and method of forming an image.
2. Discussion of the Background
Original and innovative researches and development have been made for electrophotography with various kinds of technical approaches. In the electrophotography, an image is formed by developing a latent electrostatic image formed by charging and irradiating the surface of an image bearing member with a colored toner to obtain a toner image, transferring the toner image to a transfer medium such as transfer paper, and fixing the toner image thereon with a heating roller, etc. A contact heating fixing system such as a heating roller fixing system is widely employed as the fixing system for toner.
The fixing device for use in the heating roller fixing system includes a heating roller and a pressure roller. In this fixing system, the toner image is fixed on the recording sheet by melting when a recording sheet bearing the toner image passes through a contact pressure (nip) portion of the heating roller and the pressure roller.
The viscosity of the melted toner drastically lowers, which easily causes an offset phenomenon in which toner is attached to the heating roller, or a winding phenomenon in which the recording sheet winds around and thus is unseparated from the heating roller.
A lubricant such as silicon oil is applied to the roller to prevent these phenomena. However, an oil application device is required, which leads to problems such as a cost increase and a size increase of the fixing portion.
In addition, the obtained image may shine due to the oil attached thereto or the oil attached to the image prevents writing on the image with a pen.
Another prevention method is provided in which a toner containing a releasing agent in its inside is used.
In this method, attachment of the toner to the heating roller is prevented by melted releasing agent oozed from the inside of the toner when the toner is melted. The releasing agent ideally has the following characteristics: (1) melting at a temperature as low as possible; (2) less amount of heat required for melting; (3) low melt viscosity; and (4) swift and steady move from the inside of the toner to the uppermost surface thereof. The required amount of the releasing agent is reduced and the fixing temperature is lowered when the releasing agent has these characteristics. In addition, the electricity consumed by the heating roller, etc. is saved and a large margin is obtained for improving the system speed (printing speed).
However, when a releasing agent having an extremely low melting point is used to satisfy (1), the releasing agent in the toner is possibly melted at a temperature rise in an image forming apparatus, for example, a development device, caused by the environmental change and/or driving of the devices, which causes a problem.
Therefore, the method of simply lowering the melting point of a releasing agent is limiting.
In addition, satisfying (2) greatly depends on the amount of the releasing agent contained in a toner. Excessive reduction of the amount of the releasing agent has an adverse impact on the releasing effect. To the contrary, an excessive amount of the releasing agent tends to require a large amount of heat and cause the releasing agent to be present exposed to the surface of the toner.
In addition, (3) significantly depends on the kind of the releasing agent, and a mutual function between the molecules of the releasing agent is preferably small. Thus, a releasing agent having a low polar structure is preferable.
However, a change in the polarity of the releasing agent has an impact on the compatibility and the mutual function between the releasing agent and the binder resin in the toner. Therefore, that the present status and the existing position of the releasing agent inside the toner greatly vary should be considered.
(4) is greatly dependent on the position and the existing status of the releasing agent inside the toner.
When the releasing agent is present close to the center of the inside of the toner, the releasing agent does not easily move to the surface of the toner when the toner is heated and fixed. Thus, preferably the releasing agent is evenly dispersed inside the toner or present close to the surface of the toner to a degree not to cause a problem to the development device, etc.
In addition, when the existing status (hereinafter referred to as domain) of the releasing agent is small (fine dispersion), the releasing agent is difficult to move inside the toner when melted or the extrusion effect by distortion due to the pressure is hardly obtained.
This is a disadvantage particularly when the domain is spherical. Thus, the form of the domain is preferably a stick or disc form.
A vinyl based polymeric resin and a resin having a polyester skeleton are typical examples of the resin for use in the toner.
These resins have advantages and disadvantages respectively with regard to the toner functions and characteristics such as fluidity, mobility, chargeability, fixability and image characteristics. Therefore, a mixture of both resins and/or a hybrid resin having both skeletons have been widely used in recent years.
In addition to the typical mixing, kneading and pulverization method as the toner manufacturing method, there are wet granulation methods also referred to as chemical toner methods such as a suspension method or emulsification method in which an organic solvent and an aqueous medium are used, a suspension polymerization method in which toner particles are directly obtained by controlling and polymerizing polymerizable monomer droplets, and an agglomeration method in which toner particles are agglomerated by preparing emulsified particulates.
For example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) 2005-064183 describes a toner having a polyester based resin as a core and a vinyl based resin as a covering layer. The cover layer having resin particulates prepared by an emulsification polymerization method using a surface active agent or an emulsification dispersion method using a surface active agent is formed on the surface of a colored resin particle manufactured by an emulsification dispersion method.
JOP 2004-295105 describes a toner prepared by a process in which resin particles are agglomerated in an aqueous medium. To be specific, a liquid dispersion is prepared by dispersing a resin solution formed by dissolving a polyester resin and a styrene acryl based resin in an organic solvent in an organic solvent. Thereafter, the organic solvent is removed from the liquid dispersion followed by agglomeration of resin particles in an aqueous medium.
In addition, JOP 2004-271686 describes a toner prepared by obtaining resin particulates having a particle size of 1 μm from a polyester based resin and carnauba wax in polyaddition reaction or polycondensation reaction, dispersing the resin particulates in an aqueous medium to prepare a liquid dispersion, and salt-outing/adhering the resin particulates in the liquid dispersion in the aqueous medium.
Japanese patent No. 3577390 describes a toner obtained by preparing resin particulates having a particle size of 0.9 μm from a polyester based resin and an oxidized polypropylene followed by agglomeration.
In addition, JOP H11-007156 describes a toner prepared by forming and agglomerating resin particulates having a size of from 0.4 to 0.7 μm from a polyester based resin and paraffin wax by using suspension granulation performed by introducing into an aqueous medium a liquid mixture prepared by dissolving or dispersing toner material containing a binder resin formed of multiple polyester resins having different acid values or glass transition temperatures and a coloring agent in an organic solvent.
These chemical toner methods have various kinds of advantages over the mixing, kneading and pulverization method and are suitable to control the resin structure and the existing position of a releasing agent.
However, when a releasing agent is dispersed in a vinyl based polymerizable resin, the vinyl based polymerizable resin and the releasing agent are easily compatibilized in each other, resulting in fine dispersion.
In addition, large particulates are not granulated by emulsification polymerization, resulting in a small domain.
Furthermore, a releasing agent is dissolved in a monomer in suspension polymerization, resulting in formation of a spherical domain with the toner located in the center.
Also, the releasing agent directly contained in toner particles by a polyester dissolution suspension method is known to be exposed to the surface of the toner.
The toner prepared by the chemical toner has not fully satisfied the function and characteristics suitable as the toner nor obtained excellent fixing characteristics suitable for a full color toner dealing with a high speed printing.